The most common method of removing waste species from a liquid stream is by adsorption. Such a method can be applied to water purification in a continuous operation with water flowing through a column or over a fixed bed of the solid adsorbent. Commercial ion-exchange resins and carbon black filters are examples of this approach.
The common characteristics of an efficient adsorbent include a large surface area and connected (i.e. open) porous structure for fast diffusion. Recent developments in this technical field include the incorporation of molecular recognition functional species (i.e. metal-binding ligands) onto the surface of various inorganic or organic carrier materials to achieve the selective adsorption of a particular group of ions out of the background ions. Among all the carrier materials explored in this developmental field, the synthetic silica gel is the most widely studied. This is because the synthetic nanoparticle silica contains a large amount of active silanol groups on its surface, necessary for the incorporation of metal-binding ligands, and has an exceptionally high surface area as well as open porous structure, necessary for achieving a rapid high-capacity adsorption.
Although much prior art has been developed based on the identical principle of incorporating metal-ion binding functional groups onto the surface of nanopore silica, the characteristics of the resulting silica-ligand composite products may differ significantly1,2,3,4,5,6,7 depending on the routes of processing. 1 L. Mercier and T. Pinnavaia, Adv., Mater., 9, No. 6, pp 500–503 (1997)2 L. Mercier, C. Detellier, Environ. Sci. Technol, 29. p 1316 (1995)3 M. S. Iamamoto, Y Gushikem, J. Colloid Interface Sci. 129, p 162 (1989)4 E. I. S. Andreotti, Y Gushikem, J. Colloid Interface Sci. 142, p 97 (1991)5 W. C. Moreira, Y Gushikem, O. R. Nascimento, J. Colloid Interface Sci. 150, p 115 (1992)6 U.S. Pat. No. 5,814,226, September 1998, Lawrence L. Tavlarides, Nandu Deorkar7 U.S. Pat. No. 5,817,239 October 1998, Lawrence L. Tavlarides, Nandu Deorkar.
Different processing techniques may start with silica gels similar in porosity and specific surface area (surface area per gram of silica) but could end up with products of distinctly different loading of the ligand groups. Or, two composites may contain a similar amount of loading of functional groups and yet differ considerably in adsorption efficiency.
One of the present inventors has recently developed an advancement in technology of the surface modification of low-density silica gel (CSMG) which can produce high surface area silica with extremely high loading of functional groups that increases the adsorption efficiency and capacitance of the silica adsorbent to a significantly higher level; this technology is the subject matter of U.S. application Ser. No. 09/601,888, filed Aug. 9, 2000, based on Provisional Application Ser. No. 60/074,026, filed on Feb. 9, 1998 and International PCT Application PCT/US99/02181, filed Feb. 3, 1999, the disclosures of which are incorporated herein in their entirety by reference thereto.
The present inventors recognized that a high-capacity adsorption may lead to a much higher concentrated environment of adsorbed specie on the surface of an adsorbent when compared with the specie concentration in the passing stream. Such increased specie population density on the pore surface could significantly increase the reaction rate of the adsorbed specie with other reactants existing nearby. Moreover, the change in the electronic state of adsorbed specie during chemisorptions could also affect its reaction rate favorably. The adsorbent, therefore, could function as a heterogeneous catalyst for the chemical reaction of adsorbed species. If the adsorbed waste specie can be converted to a less harmful or even useful specie by such a reaction, the adsorbent then becomes a reactive adsorbent. The additional option of in-situ reaction to convert the adsorbed specie provided by a reactive adsorbent can significantly increase its treatment capacity because the converted waste species normally do not occupy the surface adsorption sites any longer. The present invention is based, in part, on the recognition and utilization of the foregoing considerations.